The present invention relates generally to apparatus and methods for accurately determining, measuring or matching the time or phase of an input signal with the time or phase of a reference, such as done in a phase-locked loop (PLL). More particularly, the invention relates to any such apparatus or methods used in connection with any communication channel employing a known carrier wave. Yet more particularly, the invention relates to such apparatus and methods as applied to the field of matching the phase of a wobble signal recovered from a recordable/writable or re-writable Digital Versatile Disk (DVD), Compact Disk (CD), Blue-ray media and HD-DVD media very precisely to the phase of a write clock having a frequency several orders of magnitude greater than that of the wobble signal.
In certain data storage systems, including DVD media, recoding equipment and playback equipment, several signals are used to extract timing information and facilitate recording and playback of information. In the following discussion, several signals are referred to. Analogous signals are found in communication channels employing a known carrier wave, as would be understood by the skilled artisan. They are defined as follows:
Definition List 1TermDefinitionWrite clock (wclk)The write clock is a signal derived fromthe wobble signal in a fixed phase andfrequency relationship thereto and isused to time the recording/writing ofeight-to-fourteen modulation (EFM) andeight-to-fourteen modulation plus(EFMP) data signals to the media.Wobble signalThe wobble signal is a signal that variesin correspondence with a physical,sinusoidal wobble incorporated into thetracks of the media at the time ofmanufacture. The wobble signal variessinusoidally about a median, or zero,value, with a constant carrier frequency,fwobble.Digital wobble signalThe digital wobble signal is a sequenceof digital values representing samples ofthe wobble signal, taken at a sample rategreater than or equal to the Nyquist ratefor the wobble signal.Reference clockThe reference clock is a clock signalformed by dividing the write clock by aninteger divisor and having a frequencysubstantially equal to that of the wobblesignal.
A conventional PLL solution, accurately matching the phase of the recovered reference clock, to the phase of the wobble signal is shown in FIG. 1. The write clock, wclk, is divided by divider, 104, to form the reference clock, at the same frequency as the wobble signal. In DVD+R/RW media, the write clock has a frequency, fwclk, 32 times the frequency of the wobble signal; while in DVD-R/RW media, the write clock has a frequency, fwclk, 186 times the frequency of the wobble signal. The reference clock and the wobble signal are applied to a phase comparator, 101, which digitally compares the phase of its inputs with a resolution finer than the frequency of the write clock, producing a control signal, for example a voltage signal, which is applied to a controllable oscillator, for example a voltage-controlled oscillator (VCO), 102. The output of the VCO, 102, may be a high speed clock from which the resolution of the phase comparator is derived, and which is divided by divider, 103, to form the high-frequency write clock, wclk, and which is divided down further to form the reference clock.
Conventional solutions are expensive and inadequately robust because it is difficult, in general, to produces a high-frequency signal having low phase jitter by phase locking two signals of frequencies several orders of magnitude lower than the frequency of the high-frequency signal to be produced. The resulting wclk accuracy and jitter depends on the resolution employed in the phase comparator/detector. The higher the resolution of the phase comparator relative to the frequency of the wclk signal, the more accurate wclk will be derived. Because the wclk frequency is scaled with the recording speed, and further because new DVD standards and associated media support increasing recording speeds, conventional solutions would impose a very high phase comparator resolution.